Direct injection spark ignition (DISI) internal combustion engines may be operated in various modes depending upon the particular objectives to be attained at any particular time with emphasis on power output, fuel economy, and/or low emissions, for example. Operating modes may include a homogeneous mode in which the combustion chambers contain a substantially homogeneous mixture of air and fuel, or a stratified mode in which the combustion chambers contain stratified layers of different air/fuel mixtures. Stratified mode generally includes strata containing a stoichiometric air/fuel mixture nearer the spark plug with lower strata containing progressively leaner air/fuel mixtures.
Typically, there is a range of air/fuel ratios within which stable combustion can be achieved in the stratified mode, such as between 25:1 and 40:1, and a second range in which stable combustion can be achieved in the homogeneous mode, such as between 12:1 and 20:1. As such, there is typically a significant gap between the leanest air/fuel ratio of the homogeneous mode (20 in this example), and the richest air/fuel ratio of the stratified mode (25 in this example). This gap poses a number of problems in selecting an appropriate operating mode and controlling the engine.
For example, best fuel economy is often associated with highest allowable manifold pressure which may dictate an air/fuel ratio which falls within the gap and is therefore not achievable in either mode of operation. As such, the engine controller operates the engine at a richer air/fuel ratio to maintain stable combustion with a resulting lower fuel economy.
The air/fuel ratio gap between operating modes also poses control challenges to avoid limit cycle behavior in which a small variation in requested torque causes cycling between stratified and homogeneous modes which have a large variation in associated air/fuel ratios.